The invention relates to a system including at least one gyro for precise angular measurements.
In a known gyro system of that kind based on a lasergyro, the lasergyro is operated within a case through constant rotation above the lock-in rate--known as rate-bias gyro. In order to obtain information on the rotation angle of the gyro with respect to its casing, the system passes after each full rotation a photo-electric null indicator permitting to determine the 360.degree. angle with a precision within the order of magnitude of one arc second (1"), approximately. By means of a 360.degree. pulse a counter is controlled which counts the output pulses (interference patterns). If the case is kept earth-fixed, the number of gyro pulses per 360.degree. or the angle of rotation per lasergyro output pulse, i.e. the scalefactor, is obtained after deduction of the pulses due to the earth rate (DLR-Nachrichten 61, November 1990, pages 12 to 15).
Gyros present the advantage that they allow angular measurements without a locally fixed basis. As far as the angular resolution is concerned, conventional laser gyros are limited, to a value between 1.5-3". This value can at best be improved by factors 2 or 4 even if the conventional signal readout is expanded. Many applications require however resolutions by one to two orders of magnitude higher.
For precision angular measurements, turntables with an in-built digital measurement system are known. They contain an encoder with angular divisions in the form of a line grid which is read out photoelectrically. In commercial measurement systems divisions of up to 36,000 lines on the circle perimeter are realized corresponding to an angular resolution of 0.01.degree.=36". In connection with a 1024-time digital interpolation of the division interval measurement steps down to 0.035" are reached.
In this connection measurement uncertainties are due to division deviations during the manufacturing of the encoder, to adjustment errors during the installation of the encoder on the angular measurement table and to interpolation errors during signal processing.
Measurement uncertainties can be reduced by means of a self-calibrating dynamic angular measurement procedure referring to the full angle of 360.degree. as error-free normal based on the permanent rotation of the encoder. In this procedure the encoder readout furnishes a periodic signal, the phase of which is evaluated as angular measurement through comparison with a phase-fixed reference signal. Through integration (averaging) over the full circle the principle is applied that the sum of all angular segments amounts to 360.degree. and the sum of all deviations of divisions to 0.degree.. In this context it is also known to have two encoder disks with identical line grid divisions mounted on a single shaft and to have them constantly driven by a motor. The readout system of the first of these two encoder disk is fixed to a base plate on which the shaft is rotatably mounted. This readout system produces a phase-fixed reference signal. The readout system of the second encoder disk is fixed to a turntable mounted coaxially with the shaft carrying the disk of the encoder and furnishes a measurement signal with changing phases (Kontakt und Studium, Band 260, "Industrielle Winkelme.beta.technik", Expert Verlag, S. 118-122). With such a system using two encoder disks the measurement uncertainty can be reduced to 0.03". For the phase readout a 64-MHz oscillator is used allowing an angular resolution of 0.04" for the individual measurements. The resolution for the whole system is in the order of 0.01".